Normalization of matrix and array types. Conceptually, a matrix is just a 2-dimensional array. But current versions of R handle matrix and 2-D array objects differently in some cases. In R 4.0.0, matrix objects will formally inherit from the array class, eliminating such inconsistencies.
A refreshed color palette for charts. The base graphics palette for current versions of R (shown as R3 below) features saturated colors that vary considerably in brightness (for example, yellow doesn’t display as prominently as red). In R 4.0.0, the palette R4 below will be used, with colors of consistent luminance that are easier to distinguish, especially for viewers with color deficiencies. Additional palettes will make it easy to make base graphics charts that match the color scheme of ggplot2 and other graphics systems.
Read on for more, as well as a link to the rest of the changes.